Applicants"" invention relates generally to radiocommunication systems, e.g., cellular or satellite systems and, more particularly, to techniques for supporting and enhancing emergency calling procedures in such systems.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have changed the ways in which people communicate. One survey indicates that about 80% of the people who purchase mobile communication units and service subscriptions do so to enhance their personal security. Presumably, many of these subscribers would expect to use their mobile units to aid them in urgent situations, e.g., when their vehicle has become disabled or in an emergency situation requiring rapid medical and/or police response. In these circumstances it would be desirable that the radiocommunication system be able to independently determine a location of the mobile unit, particularly in the case where the subscriber does not know his or her precise location. Moreover, it is expected that the FCC will soon require that network operators forward the position of an emergency caller to the emergency service provider.
There are many techniques available to generate mobile unit location information. In a first category, the mobile unit could estimate its own position and send a message with its coordinates when placing an emergency call. This could be accomplished by, for example, providing the mobile unit with a Global Positioning System (GPS) receiver that receives location information from the GPS satellite network. The mobile unit can then transmit this information to the system, which would then forward it to the emergency service provider. This requires, however, significant modification of existing mobile units to include GPS receivers, as well as additional signalling between the mobile units and base stations.
Alternatively, the base stations which transmit signals to, and receive signals from, the mobile units could be used to determine the mobile unit""s location. Various techniques, including attenuation of a mobile unit""s signal, angle-of-arrival, and difference between the time-of-arrival (TDOA) of a mobile unit""s signal at different base stations, have been suggested for usage in providing mobile unit location information. See, for example, the article entitled xe2x80x9cTime Difference of Arrival Technology for Locating Narrowband Cellular Signalsxe2x80x9d by Louis A. Stilp, SPIE Vol. 2602, pp. 134-144. These solutions also have their drawbacks including the need to modify the many existing base stations, e.g., to provide array antennas to support angle-of-arrival techniques or to synchronize base station transmissions to support TDOA techniques.
A third category of strategies for locating mobile units in radiocommunication systems involves the provision of an adjunct system, i.e., a system which may be completely independent of the radiocommunication system or which may share various components (e.g., an antenna) with the radiocommunication system but which processes signals separately therefrom. This may be advantageous, for example, as an expedient solution to providing mobile unit location without modifying the large number of existing base stations in a system. For example, consider the equipment illustrated in FIG. 1 wherein the adjunct scanning units are not co-located with the base stations of radiocommunication system. Therein, a base station 1 supports radiocommunication within cell 2 and, in particular with mobile unit 3. An adjunct system, partially shown by way of scanning units 4, 5 and 6, monitors accesses to the system by mobile unit 3. When mobile unit 3 makes an emergency access, adjunct units 4, 5 and 6 can detect this emergency access by, for example, the presence of a set emergency flag in an origination message or based upon the dialed number. The adjunct units can then use the mobile unit""s transmissions on either a control channel or a traffic channel to provide information to a location processing center 7. The location processing center then uses the information provided by the various adjunct units to, for example, triangulate the position of mobile unit 3 and report this position to an emergency service center 8. More details regarding exemplary usages of adjunct systems can be found in U.S. Pat. No. 5,327,144 to Stilp et al., entitled xe2x80x9cCellular Telephone Location Systemxe2x80x9d, the disclosure of which is incorporated here by reference.
If such an adjunct system is overlaid with existing radiocommunication systems, e.g., cellular systems, it may be possible to provide certain adjustments to the operation of the radiocommunication system which enhance operation of the adjunct system. Similarly, the radiocommunication system may be able to take advantage of the adjunct system to enhance its performance.
According to one exemplary embodiment of the present invention, position information generated by an adjunct system is used by the radiocommunication system to avoid localized congestion caused by many mobile units reporting the same emergency. For example, if a traffic peak occurs, the radiocommunication system can evaluate incoming emergency calls using the position information generated by the adjunct system. If a number of emergency calls are received from the same location when the system is relatively congested, then the radiocommunication system can reduce the priority of incoming emergency calls from that location. Similarly, the position information generated by the adjunct system can be used by the radiocommunication system for other purposes, e.g., dynamic channel allocation.
According to another exemplary embodiment of the present invention, the radiocommunication system can render it easier for the adjunct system to refine its position estimate for a mobile unit and/or to follow a mobile unit to its assigned traffic channel for an emergency call. This can be accomplished by repeating the transmission of a traffic channel assignment message on the control channel which the mobile unit is using to make the emergency access.
According to another exemplary embodiment of the present invention, the mobile unit can also adopt certain practices in response to its placement of an emergency call. For example, it can enter a mode in which its transmissions are tailored to aid the adjunct system, e.g., transmitting with maximum or high power, having no encryption of its data messages, or continuously transmitting. Alternatively, the mobile unit can enter a low power mode wherein it reduces battery drain (e.g., by entering discontinuous transmission mode, using lower transmit power, etc.) and transmits an indication of the amount of battery power/talk time left.